Exhaust smoke-processing system

ABSTRACT

An exhaust smoke processing system capable of economically removing heavy metal, comprising a air preheater  3  for heating combustion air by exhaust smoke discharged from a boiler  1,  a heat recoverer  11  for heating a heat medium by exhaust smoke discharged from the air preheater  3,  a dust collector  4  for collecting soot and dust in exhaust smoke discharged from the heat recoverer  11,  a wet-type exhaust smoke processing apparatus for processing exhaust smoke discharged from the dust collector  4,  a reheater  13  for heating exhaust smoke discharged from the wet-type exhaust smoke processing apparatus by the heat medium, and a heat medium circulation pipe passage  15  for circulating the heating medium between the reheater  13  and the heat recoverer  11,  wherein the heating medium circulation pipe  15  is provided with temperature control means which measures a heavy metal concentration in exhaust smoke discharged from any one or more of the dust collector  4,  the wet-type exhaust smoke processing apparatus  6  and the reheater  13,  and adjust the temperature of exhaust smoke at an outlet of the heat recoverer  11  such that the measured value falls within a predetermined range.

TECHNICAL FIELD

The present invention relates to an exhaust smoke processing system, andmore particularly, to an exhaust smoke processing system in which inorder to reduce the heavy metal concentration in exhaust smokedischarged from a smokestack, a heat recoverer for reheating exhaustsmoke discharged from a wet-type exhaust smoke processing apparatus isdisposed upstream of a dust collecting apparatus such as a bag filter oran electrostatic precipitator, thereby lowering the temperature of theexhaust smoke at an inlet of the electrostatic precipitator.

BACKGROUND TECHNIQUE

A fossil fuel such as coal includes heavy metal which is harmful to ahuman body although an amount thereof is very small. When the fossilfuel is burned, most of them become gas. In a thermal power plant or thelike, after most of nitrogen oxide (NOx), sulfur oxide (SOx) and smokeand dust in the exhaust smoke generated when coal or the like is burnedis removed, the remainder thereof is discharged to atmosphere from asmokestack. However, a portion of the heavy metal is not completelyremoved by a dust collecting aaparatus which collects smoke and dust inthe exhaust smoke or by a desulfurization apparatus which removes sulfuroxide. Examples of elements of heavy metal included in coal which areeasily discharged from smokestack are mercury, selenium, arsenic,chromium, lead and the like having high volatility. Although theconcentration of these elements in the exhaust smoke when they aredischarged from the smokestack is not high, it is desired that itsprocessing technique becomes widespread, since their toxicity is high.For example, there is proposed a mercury processing method in theexhaust gas discharged from a refuse incinerator as a method forremoving mercury in the exhaust gas (Japanese Patent Publications Nos.H6-61424 and H6-104182). This is because that higher concentrationmercury is included in exhaust gas from the refuse incinerator.

According to the techniques disclosed in the above publications, liquidabsorbent or solid absorbent is sprayed into exhaust gas, mercury in theexhaust gas is collected by the absorbent, and the absorbent whichcollected the mercury together with dust in the exhaust gas is collectedby a dust collecting apparatus such as a downstream bag filter or aelectrostatic precipitator. It is proposed that powder activated carbonis sprayed upstream of a bag filter which removes mercury, and it iscollected by a downstream bag filter (Felsvang K. et al.: Activatedcarbon injection in spray dryer/ESP/FF for mercury and toxics control:Fuel Process Tech. 39 PP. 417-430 (1994)).

According to these conventional techniques, however, an apparatus whichsprays liquid absorbent or solid absorbent into exhaust gas, and areactor which brings liquid absorbent or solid absorbent and exhaust gasinto contact for a certain time (normally several seconds) are required,and it is necessary to secure the installation space therefore. Further,liquid absorbent or solid absorbent is expensive., and there is aproblem that the processing cost is increased.

That is, according to the conventional techniques, consumption amount ofexpensive liquid absorbent or solid absorbent is large, and the reactorand the apparatus for spraying liquid absorbent or solid absorbent intoexhaust gas are required, and it is necessary to secure the installationspace therefore. It is an object of the present invention to propose anexhaust smoke processing system capable of solving these problems andcapable of economically removing heavy metal.

DISCLOSURE OF THE INVENTION

The object of the present invention can be achieved by the followingmeans. That is, the exhaust smoke processing system comprises an airpreheater for heating air for combustion by exhaust smoke dischargedfrom a boiler, a heat recoverer for heating a heat medium by exhaustsmoke discharged from the air preheater, a dust collector for collectingsoot and dust in exhaust smoke discharged from the heat recoverer, awet-type exhaust smoke processing apparatus for wet-type processingexhaust smoke discharged from the dust collector, a reheater for heatingexhaust smoke discharged from the wet-type exhaust smoke processingapparatus by the heating medium, and a heat medium circulation pipe forcirculating the heating medium between the reheater and the heatrecoverer. The heat medium circulation pipe passage is Provided withtemperature control means which measures a heavy metal concentration inexhaust smoke discharged from any one or more of the dust collector, thewet-type exhaust smoke processing apparatus and the reheater, and adjustthe temperature of exhaust smoke at an outlet of the heat recoverer suchthat the heavy metal concentration falls within a predetermined range.With this, even if composition of coal to be burned or the temperatureof exhaust smoke at the inlet of the heat recoverer is varied, the heavymetal concentration in exhaust smoke discharged into the atmosphere canfall within the predetermined range. In this case, the temperaturecontrol means can be realized by using any one of or more of means foradjusting a heat medium circulation flow rate of the heat mediumcirculating between the reheater and the heat recoverer, means forcooling the heat medium, means for heating the heat medium, and meanswhich disposes a bypass pipe for connecting an inlet and an outlet of apassage of the heating medium flowing into the heat recoverer and whichadjusts a flow rate of the heat medium in the bypass pipe. A methoddescribed in Japanese Patent Applications Laid-open Nos. H9-122438 andH11-347332 can be used as the means for controlling the temperature ofexhaust smoke at the outlet of the heat recoverer.

Further, the present invention provides an exhaust smoke processingsystem comprising an air preheater for heating combustion air by exhaustsmoke discharged from a boiler, a dust collector for collecting soot anddust in exhaust smoke discharged from the air preheater, and a wet-typeexhaust smoke processing apparatus for wet-type processing exhaust smokedischarged from the dust collector. This system further comprisescontrol means which measures a heavy metal concentration in exhaustsmoke discharged from the wet-type exhaust smoke processing apparatus,and which adjusts any one or more of pH of liquid absorbent of thewet-type exhaust smoke processing apparatus, a flow rate ofoxidizing-air, and a flow rate of waste water, such that the heavy metalconcentration falls within a predetermined range.

In this invention, most of heavy metal in the exhaust smoke exists asgas in a high temperature region in the boiler, but if the temperatureof the exhaust smoke is lowered, the heavy metal becomes prone to beattached to a surface of solid particle such as ash particle. This isbecause that as the temperature of the element and compound becomeslower, the vapor pressure becomes lower also, and the heavy metal cannot easily exist as gas. Therefore, in the case of the dust collectingapparatus which collects smoke and dust in exhaust smoke, as thetemperature of the exhaust smoke is lower, the apparatus can collectmore the heavy metal together with ash particle. It is possible toremove and collect the heavy metal in the collected ash as required, orto stabilize the corrected ash such that the heavy metal is not elutedfrom the ash particle. However, if the temperature of the exhaust gas inthe dust collecting apparatus becomes excessively low, the ash particlebecomes prone to be agglutinated, and there is a problem that the ashparticle can not easily be discharged from an ash collecting section(hopper) provided in a lower portion of the dust collecting apparatus.Therefore, if the heavy metal concentration in exhaust smoke is measuredunder the temperature condition of the exhaust smoke capable of stablycollecting ash from the dust collecting apparatus, and if thetemperature of exhaust smoke at the outlet of the heat recoverer (inletof the dust collecting apparatus) is adjusted such that the componentconcentration falls within the predetermined range, it is possible tocontrol the concentration of heavy metal discharged from the smokestackinto the atmosphere.

The exhaust gas including heavy metal which was not removed by the dustcollecting apparatus enters the downstream wet-type exhaust smokeprocessing apparatus, and sulfur dioxide gas (SO₂) in the exhaust gas isremoved by the liquid absorbent. At that time, a portion of the heavymetal is absorbed by the liquid absorbent. The present inventorsresearched and found that pH of liquid absorbent, the flow rate ofoxidizing-air and concentration of heavy metal in the liquid absorbentaffect the removing efficiency of the heavy metal. That is, as the pHand the heavy metal concentration in the liquid absorbent are lower, oras the flow rate of oxidizing-air is higher, the removing efficiency ofheavy metal by the wet-type desulfurization apparatus becomes higher.Therefore, when the heavy metal concentration in the exhaust smokedischarged into the atmosphere is high, the problem can be solved bylowering pH of liquid absorbent, increasing the amount of waste water tobe discharged, or increasing the oxidizing-air. In this manner, it ispossible to control the heavy metal concentration discharged from thesmokestack into the atmosphere by measuring the heavy metal component inexhaust smoke discharged from the wet-type desulfurization apparatus,and by adjusting any one or more of pH of liquid absorbent of thewet-type exhaust smoke processing apparatus, a flow rate ofoxidizing-air, and a flow rate of waste water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a basic structure of a system of the present invention;

FIG. 2 shows a detailed structure of the system of the invention; and

FIG. 3 shows a detailed structure of the system of the invention.

Explanation of symbols: 1 . . . boiler, 2 . . . denitration apparatus, 3. . . air preheater, 4 . . . electrostatic precipitator, 5 . . . induceddraft fan, 6 . . . wet-type desulfurization apparatus, 7 . . .desulfurization fan, 8 . . . smokestack, 9 . . . measuring apparatus 10. . . pump, 11 . . . heat recoverer, 12 . . . heat-transfer piping, 13 .. . reheater, 14 . . . heat-transfer piping, 15 . . . heat mediumcirculation pipe passage, 16 . . . heat medium bypass line, 17 . . .thermometer, 18 . . . flow regulating valve, 19 . . . heat exchanger, 20. . . supply pipe, 21 . . . flow regulating valve, 31 . . . inlet duct,32 . . . desulfurization tower body, 33 . . . outlet duct, 34 . . .liquid absorbent circulation line, 35 . . . pump, 36 . . . spray nozzle,37 . . . circulation tank, 38 . . . agitator, 39 . . . air supply pipe,40 . . . limestone slurry tank, 41 . . . pump, 42 . . . limestone supplypipe, 43 . . . pH meter, 44 . . . valve, 45 . . . plaster drawing-outpipe, 46 . . . dewater, 47 . . . return piping, 48 . . . waste pipe.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be explained in more detail by way ofExamples.

EXAMPLE 1

FIG. 1 is a block diagram of an exhaust smoke processing systemaccording to an embodiment of the invention. According to the exhaustsmoke processing system of this embodiment, exhaust smoke dischargedfrom a boiler 1 is introduced into a denitration apparatus 2. Here,nitrogen oxide in exhaust smoke is removed and then, the exhaust smokeis introduced into an air preheater 3. The exhaust smoke introduced intothe air preheater 3 is heat-exchanged with combustion air supplied tothe boiler 1 to be cooled to 120 to 155° C., for example, and isintroduced into a heat recoverer 11. Heat of exhaust smoke introducedinto the heat recoverer 11 is recovered in heat medium flowing in aheat-transfer pipe by heat exchange, the exhaust smoke is cooled to 75to 110° C., for example and is introduced into an electrostaticprecipitator 4. Here, most of smoke and dust in the exhaust smoke iscollected. The exhaust smoke which passed through the electrostaticprecipitator 4 is increased in pressure by an induced draft fan 5, andis introduced into a wet-type desulfurization apparatus 6 using aspray-type limestone-plaster method, for example, wherein Sox in theexhaust smoke is removed by gas-liquid contact. The exhaust smoke whichwas cooled to a saturated gas temperature in the wet-typedesulfurization apparatus 6 is increased in temperature in a reheater13, and the exhaust smoke is discharged from a smokestack 8 through adesulfurization fan 7. Like the heat recoverer 11, the reheater 13 is aheat exchanger having a heat-transfer pipe through which heat mediumflows. The exhaust smoke is heat-exchanged with the heat medium flowingthrough the heat-transfer pipe to be increased in temperature to 90 to110° C., for example. The heat recoverer 11 and the heat-transfer pipeof the reheater 13 are in communication with each other through heatmedium circulation pipe passages 15-1, 15-2, wherein the heat medium iscirculated between the heat recoverer 11 and the reheater 13 by a pump10. The heavy metal concentration in exhaust gas at an inlet of thesmokestack 8 is measured by a measuring apparatus 9, and the temperatureof exhaust gas at outlet of the heat recoverer 11 (inlet of the electricdust collector 4) is controlled based on the measured value.

In this manner, according to the exhaust smoke processing system of thepresent invention, the removing efficiency of heavy metal in the exhaustgas can be enhanced by controlling the temperature of exhaust gas atoutlet of the heat recoverer 11 (inlet of the electric dust collector4).

FIG. 2 shows details of heat medium circulating system of the heatrecoverer and the reheater according to the feature of the presentinvention. The heat-transfer piping 12 of the heat recoverer 11 and theheat-transfer piping 14 of the reheater 13 are annularly connected toeach other through the heat medium circulation pipe passages 15-1, 15-2,and the circulation pump 10 is provided at an intermediate portion ofthe passages. The heat medium is circulated in the heat-transfer pipings12 and 14 by the circulation pump 10. The heat-transfer pipings 12 and14 use fin tubes to enhance the heat exchange efficiency. To absorb theexpansion of the heat medium in the passage, heat medium tank isdisposed.

A concrete method for controlling the temperature of exhaust gas at theoutlet of the heat recoverer 11 (inlet of the electric dust collector 4)will be explained below.

A heat medium bypass line 16 is provided for controlling the temperatureof exhaust smoke at the outlet of the heat recoverer 11. A thermometer17 measures the temperature of exhaust smoke at the outlet of the heatrecoverer 11. An opening of a flow regulating valve 18 is adjusted tocontrol the heat collecting amount such that the temperature of theexhaust smoke at the outlet of the heat recoverer 11 becomes equal to orhigher than a set value by a signal of the thermometer 17. A heatexchanger 19 is provided in the heat medium circulation pipe passage15-2. Vapor or cooling water is allowed to flow to the heat exchanger 19from a supply pipe 20 while adjusting the opening of the flow regulatingvalve 21, thereby controlling the temperature of the exhaust smoke atthe outlet of the heat recoverer 11.

It is also possible to control the temperature of the exhaust smoke atthe outlet of the heat recoverer 11 by adjusting the flow rate of thepump 10. For example, when the value measured by the measuring apparatus9 is higher than a predetermined value, the flow rate of the pump 10 isincreased, the amount of heat exchange between the heat recoverer 11 andthe reheater 13 is increased, and the temperature of the exhaust smokeat the outlet of the heat recoverer 11 (inlet of electric dust collector4) is reduced. However, if the flow rate of the pump 10 is decreased toreduce the heat exchange amount between the heat recoverer 11 and thereheater 13, the temperature of exhaust gas at the outlet of thereheater 13 is lowered, mist scattered from the wet-type desulfurizationapparatus sticks to a surface of the heat-transfer pipe 14, and this maycause corrosion.

In the example shown in FIGS. 1 and 2, the temperature of the exhaustsmoke at the outlet of the heat recoverer 11 (inlet of electric dustcollector 4) was adjusted to a predetermined value, and the heavy metalconcentration in the exhaust gas at the inlet of the smokestack 8 wasmeasured by the measuring apparatus 9. Table 1 shows a result of theabove measurement. In Table 1, relative values based on the values at80° C. are shown. TABLE 1 Result of analysis of heavy metalconcentration in exhaust gas (−) Exhaust gas temperature (° C.) 80 100120 140 Mercury 1.0 1.5 2.5 3.5 Selenium 1.0 1.4 2.2 3.0

EXAMPLE 2

In the exhaust smoke processing system shown in FIG. 1, it is alsopossible to control the heavy metal concentration in exhaust gasdischarged into the atmosphere, by adjusting one or more of operationconditions of the desulfurization apparatus 6, pH of liquid absorbent,oxidizing-air flow rate, and waste water flow rate.

FIG. 3 shows a detail structure of the desulfurization apparatus 6.Exhaust gas including heavy metal that was not removed by the dustcollecting apparatus (not shown) is introduced into a desulfurizationtower body 33 from an inlet duct 31 and is discharged from an outletduct 32. During that time, liquid absorbent sent by a pump 35 through aliquid absorbent circulation line 34 is sprayed from a plurality ofspray nozzles 36 in the desulfurization tower, and the liquid absorbentand the exhaust gas come into contact with each other in a liquid/gascontact manner. At that time, the liquid absorbent absorbs SO₂ in theexhaust gas and produces calcium sulfite. The liquid absorbent thatproduced calcium sulfite is accumulated in a circulation tank 37 andstirred by an agitator 38 and in this state, calcium sulfite in theliquid absorbent is oxidized by air supplied from an air supply pipe 39,to produce plaster. Desulfuirizer such as limestone is added to theliquid absorbent in the circulation tank 37 through a limestone supplypipe 42 by a pump 41 from a limestone slurry tank 40. An amount oflimestone to be supplied is adjusted by a valve 44 based on aninstructed value of a pH meter 43 disposed in the liquid absorbentcirculation line 34. A portion of the liquid absorbent in the tank wherelimestone and plaster coexist is sent through a plaster drawing-out pipe45 to a dewater 46, where the plaster is recovered. A portion offiltrate of the dewater is returned into the circulation tank 37 throughthe return piping 47, and the remainder is discharged from the wastepipe 48 to outside of the system as waste water.

When pH of liquid absorbent of the desulfurization apparatus shown inFIG. 3 was changed, a mercury concentration change in exhaust gas at theoutlet of the desulfurization apparatus was measured. The result isshown in Table 2, where relative values based on values at pH4.5 areshown. As pH is lower, the mercury concentration in exhaust gas at theoutlet of the desulfurization apparatus is lower. However, if pH ofliquid absorbent becomes lower, as desulfurization performance isdeteriorated, it is necessary to increase a liquid and gas ratio (L/G).TABLE 2 Result of analysis of heavy metal mercury concentration inexhaust gas (−) pH 4.5 5.0 5.5 6.0 Mercury 1.0 2.5 3.5 5.0

In the system diagram of FIG. 3, calcium sulfite in liquid absorbent isoxidized in the circulation tank 37 by air supplied from the air supplypipe 39 to generate plaster. At that time, if the amount ofoxidizing-air is not sufficient, calcium sulfite remains, and theremoving rate of heavy metal is lowered. A mercury concentration changein exhaust gas at the outlet of the desulfurization apparatus wasmeasured when a sulfur dioxide concentration in liquid absorbent waschanged. The result is shown in Tale 3, where relative values based onvalues of sulfur dioxide concentration 0.0 (mmol/L) are shown. There isa tendency that as the sulfur dioxide concentration becomes higher, themercury concentration in exhaust gas at the outlet of thedesulfurization apparatus becomes higher. TABLE 3 Result of analysis ofheavy metal mercury concentration in exhaust gas (−) Sulfur dioxideconcentration (mmol/L) 0.0 1.0 2.0 3.0 Mercury 1.0 2.0 4.5 6.0

As shown in the system diagram of FIG. 3, the desulfurization liquidabsorbent is sent to the dewater 46, where plaster is recovered, and aportion of the filtrate is returned into the circulation tank 37 throughthe return piping 47. A portion of the heavy metal removed in thedesulfurization apparatus is concentrated in the liquid absorbent. Themercury concentration change in exhaust gas at the outlet of thedesulfurization apparatus was measured when the mercury concentration ofthe liquid absorbent was changed. The result is shown in Table 4, whererelative values based on values of mercury concentration 0.03 (mmol/L)are shown. As the heavy metal concentration in liquid absorbent becomeshigher, its removing performance becomes lower. Therefore, in order toenhance the removing performance, it is necessary to increase the amountof water discharged from the system from the waste pipe 48. TABLE 4Result of analysis of heavy metal mercury concentration in exhaust gas(−) Mercury concentration (mmol/L) 0.03 0.1 0.2 0.3 Mercury 1.0 2.5 3.54.5

The problem that the heavy metal concentration in exhaust gas dischargedinto atmosphere is high, can be solved by lowering pH of liquidabsorbent, increasing the amount of water to be discharged, andincreasing the oxidizing-air.

Although the electrostatic precipitator is used in this embodiment, abag filter can be used instead thereof.

Although it is not described in the embodiment, in order to remove solidparticle such as ash particle sticked to a surface of the heat-transferpipe 12 of the-heat recoverer 11 shown in FIGS. 1 and 2, an apparatus(soot blower) which blows air or vapor to remove the particle isdisposed. If the soot blower is used, the smoke and dust concentrationin exhaust gas is temporarily increased and with this, the heavy metalconcentration in the exhaust gas is also increased. Therefore, it ispreferable to take the timing for operating the soot blower intoconsideration.

INDUSTRIAL APPLICABILITY

The present invention can economically provide an exhaust smokeprocessing system in which the heavy metal concentration in exhaustsmoke discharged from smokestack is reduced.

1. An exhaust smoke processing system comprising an air preheater forheating combustion air by exhaust smoke discharged from a boiler, a heatrecoverer for heating heat medium by exhaust smoke discharged from theair preheater, a dust collector for collecting soot and dust in exhaustsmoke discharged from the heat recoverer, a wet-type exhaust smokeprocessing apparatus for wet-type processing exhaust smoke dischargedfrom the dust collector, a reheater for heating exhaust smoke dischargedfrom the wet-type exhaust smoke processing apparatus by the heat medium,and a heat medium circulation pipe passage for circulating the heatmedium between the reheater and the heat recoverer, wherein the heatmedium circulation pipe passage is provided with temperature controlmeans which measures a heavy metal concentration in exhaust smokedischarged from any one or more of the dust collector, the wet-typeexhaust smoke processing apparatus and the reheater, and adjust thetemperature of exhaust smoke at an outlet of the heat recoverer suchthat the heavy metal concentration falls within a predetermined range.2. The exhaust smoke processing system according to claim 1, wherein thetemperature control means is any one of or more of means for adjusting aheat medium circulation flow rate of the heat medium circulating betweenthe reheater and the heat recoverer, means for cooling the heat medium,means for heating the heat medium, and means which disposes a bypasspipe for connecting an inlet and an outlet of a passage of the heatmedium flowing into the heat recoverer and which adjusts a flow rate ofthe heat medium in the bypass pipe.
 3. An exhaust smoke processingsystem comprising an air preheater for heating combustion air by exhaustsmoke discharged from a boiler, a dust collector for collecting soot anddust in exhaust smoke discharged from the air preheater, and a wet-typeexhaust smoke processing apparatus for wet-type processing exhaust smokedischarged from the dust collector, wherein the system further comprisescontrol means which measures a heavy metal concentration in exhaustsmoke discharged from the wet-type exhaust smoke processing apparatus,and which adjusts any one or more of pH of liquid absorbent of thewet-type exhaust smoke processing apparatus, a flow rate ofoxidizing-air, and a flow rate of waste water, such that the heavy metalconcentration falls within a predetermined range.
 4. An exhaust smokeprocessing system comprising a air preheater for heating combustion airby exhaust smoke discharged from a boiler, a heat recoverer for heatinga heat medium by exhaust smoke discharged from the air preheater, a dustcollector for collecting soot and dust in exhaust smoke discharged fromthe heat recoverer, a wet-type exhaust smoke processing apparatus forwet-type processing exhaust smoke discharged from the dust collector, areheater for heating exhaust smoke discharged from the wet-type exhaustsmoke processing apparatus by the heat medium, and a heat mediumcirculation pipe passage for circulating the heat medium between thereheater and the heat recoverer, wherein the system further comprisescontrol means which measures a heavy metal concentration in exhaustsmoke discharged from the dust collector, and adjusts the temperature ofexhaust smoke at an outlet of the heat recoverer such that the heavymetal concentration falls within a predetermined range, and which alsomeasures the heavy metal concentration in exhaust smoke discharged fromthe wet-type exhaust smoke processing apparatus, and adjusts any one ormore of pH of liquid absorbent of the wet-type exhaust smoke processingapparatus, a flow rate of oxidizing-air, and a flow rate of waste water,such that the heavy metal concentration falls within a predeterminedrange.